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Recommendations to troubleshoot brake fluid leak

My '95 525i wth 188K milles is leaking brake fluid. I get the "low brake fluid" message about once a month and would have to fill it up. Please advise as to how to best trouble shoot. I can't really tell if it is the master cylinder leaking or the brake fluid reservoir? Thanks in advance.

Re: Recommendations to troubleshoot brake fluid leak

Do you have a reason to suspect only these two areas? There are several other places a brake system can leak, including each caliper, hose, joint and fitting. It might even be the clutch cylinder(s) if you have a manual tranny. The first step to finding a leak is to get all the parts clean. Use a good engine cleaner and clean everything. An old mechanic trick is once everything is clean, coat everything with a thin dusting of talcum powder. This will let you pinpoint any leak very quickly and it washes away easily.

Re: Recommendations to troubleshoot brake fluid leak

Thank you Big Dog.

When my car was in the shop to replace the radiator, I had my shop which mostly performs modifications only put my car on the lift and look underneath to check if the brake lines and calipers were leaking. After our inspections, we determine they (brake lines and calipers) were not leaking at all.

He suspects that it may be the master cylinder since there was evidence of brake fluid residue. He also suggested to replace the brake reservoir. I was considering to replace them but I want to be sure it fixes the root cause and not replace unnecessary parts.

Re: Recommendations to troubleshoot brake fluid leak

You should see evidence of any leakage if at the master cylinder, otherwise the area should be dry. Common leakages are at the rubber grommets where the brake reservoir snap onto the master cylinder. This was my case on my 95 530. Just replace with new (2) grommets, it is an easy 15 minute DYI. Also check the reservoir for cracks.

Re: Recommendations to troubleshoot brake fluid leak

Not a problem.

First wipe clean the area around the cap then unscrew and remove the cap from the reservoir, have a small container (open top can or a plastic bag) and place the cap with the brake level sensor into it catch any brake fluid drips.

Next empty all of the brake fluid from the reservoir using one of those turkey baster and discard the old fluid into a bottle. Be very careful with the old fluid, as it is corrosive and will eat up your paint. Cover the surround area carefully.

Removal of the old reservoir is a snap, just pull the reservoir straight up, you may feel a little resistance, but keep pulling straight up slowly and itíll come off the grommets. Do not jerk, you don't want any fluid to be flying.

With the reservoir empty and removed, clean the reservoir with a clean cloth, especially the two nozzles on the bottom, take care not to leave any lints behind. Remove the old grommets from the master cylinder using a pair of needle pliers or ply loose with a small flat blade screw driver, take care not to gouge the master cylinder, next insert the new grommets by hand (not the pliers to avoid damage to new grommets), just push the grommets into place and reinstall the reservoir by lining up the nozzles on the bottom with the new grommets and gently push down to sit the reservoir. Use new brake fluid (I believe DOT 4? - check your owners manual) and fill slowly to avoid trapping any air bubbles. Replace the cap and sensor.

Final step is to test the brake system by pumping the brake pedal a few times, it should become gradually hard to push on the pedal. Youíre done. If after a few pumps on the pedal and you still able to push the pedal to the floor, then you have air in your system and will need to be bled to get the air out.

Re: Recommendations to troubleshoot brake fluid leak

Yep, use DOT 4 and you should not have to bleed the system if your careful. The grommets are propably leaking with the age and mileage, common leak. If you look closely under the resovouir you will see paint damage if they have been leaking.